You can fit better antennas to the router and sometimes the cards in the PC. They cost about £4-5 a pop. The other things to look out for are cordless phones as they run on 2.4Ghz and therefore your wifi takes a hit when they are in use. Microwave ovens can also interfere.

Download netstumbler and use it to locate neighbouring networks it could be someone has set up a new one on the same channel as yours (probably 6 or 11). Change your network to a new channel that no one else is using and the problem might disapear.

It could be that a neighbour has a new wireless router. If this is on the same channel as yours then it could interfer with your signal (default tends to be 1).

Try changing the channel on the router and see if it improves.

There are some tools oput there on tinter web that you can use to see what routers are in range and what settings they have. Could use this tio find an unused channel that is as far from others as possible